A 68-year-old man is evaluated in the intensive care unit for decreased urine output. Three days ago, he was admitted with severe community-acquired pneumonia and septic shock, which required aggressive intravenous fluid resuscitation and continuous norepinephrine infusion. Over the past 24 hours, his urine output has decreased to 250 mL. His blood pressure is 105/65 mm Hg, pulse is 98/min, and respirations are 18/min. Physical examination reveals warm extremities and trace bilateral pitting edema. Laboratory studies are obtained:
| Parameter | Value |
|---|---|
| Serum sodium | 140 mEq/L |
| Serum potassium | 4.8 mEq/L |
| Serum chloride | 102 mEq/L |
| Serum bicarbonate | 20 mEq/L |
| Blood urea nitrogen | 42 mg/dL |
| Serum creatinine | 2.8 mg/dL |
| Urine sodium | 50 mEq/L |
| Urine creatinine | 25 mg/dL |
Based on these findings, what is this patient's calculated fractional excretion of sodium (in percentage)?
Answer: 4 %
Answer
4
The fractional excretion of sodium (FeNa) is calculated using the formula . By substituting the patient's laboratory values ( = 50 mEq/L, = 2.8 mg/dL, = 140 mEq/L, = 25 mg/dL) into the equation, the result is , which equals 4%. A FeNa > 2% in the setting of oliguric acute kidney injury strongly suggests intrinsic damage, such as acute tubular necrosis, which aligns with this patient's history of prolonged septic shock and ischemic injury.
Step-by-Step Solution
Key Concept
Calculation and interpretation of the Fractional Excretion of Sodium (FeNa) in acute kidney injury.